Green MEP Eva Joly, who is said to be considering a bid for the French presidency. Photograph: Gerard Julien/AFP

Eva Joly does not look like Europe's most successful fraud prosecutor, the scourge of French boardrooms for decades. She is more than half an hour late as she sweeps into a cafe in Montparnasse from a cold, wet, grey Paris morning. Her eyes smile over natty, red-rimmed spectacles as she introduces herself in quiet tones, holding on to a handshake throughout, and apologises for her delayed arrival. Her French accent shows no trace of her Norwegian roots.

Now a leading figure in the French Green party and an MEP, Joly is expected to run for president next year. To many French voters, though, she remains best known for her extraordinary eight-year investigation into the affairs of oil multinational Elf in the 1990s. It rocked France's political, judicial and corporate elites as the unwavering Joly peeled away layer after layer of fraud — from initial questions about a Gabonese subsidiary to myriad allegations of fraud and corruption at the highest level at home and abroad.

For her pains, Joly had to spend long periods shadowed by bodyguards after threats were made against her. Her successes caused such a stir that the director Claude Chabrol loosely based his film A Comedy of Power (L'ivresse du Pouvoir) on her battles with the French establishment.

Despite her 67 years, she is bristling with the same drive and determination, underpinned by a rare blend of political zeal and a lawyer's attention to detail. "I am not getting away from combating fraud," she says. "This is why I am going into politics. It's another way of fighting fraud in the interests of ordinary people, trying to prevent the oligarchy taking all the power and all the money … We have been living in a kind of collective illusion that this is how the world is — that the poor shall always be poor, ordinary people shall be squeezed to the benefit of a kind of oligarchy."

Polemical and incendiary asides follow thick and fast. She talks about Joseph Cassano, the former financial engineering executive at bailed-out US insurer AIG ("What kind of good does he do humanity? None"); WikiLeaks ("It shows it is possible to tell people what really happened. This is prevention of fraud"); of the late Nigerian dictator Sani Abacha ("He laundered millions, and he used London. The banks that received this money are getting away with it"); and of the UK's non-dom tax arrangements for the super-rich ("This is a serious hole in European tax co-operation").

Meltdown

Her late arrival means much of the interview must be carried out in a taxi as she races across Paris to catch a train to Brussels, where she is campaigning for tougher EU action against tax havens. Later in the week she will fly to Tunisia to offer advice on how a new regime might track down the assets of the former president Zine al-Abidine Ben Ali, who fled amid popular uprising and corruption allegations last month.

While her focus now is increasingly returning to France and the political arena, recent years have seen Joly's attentions extend to Iceland, where she was recruited to advise the government's special prosecutor, Ólafur Hauksson, on criminal investigations triggered by the tiny country's spectacular financial meltdown in the autumn of 2008.

So large and far-reaching are allegations of fraud and corruption that the investigation has been described as the largest white-collar crime inquiry ever attempted anywhere in the world. Much of the suspect behaviour took place in London, and may have been unwittingly financed by hundreds of thousands of UK retail depositors, who rushed to set up online deposit accounts with Icelandic banks four years ago. The Serious Fraud Office is working closely with Hauksson's team.

This inquiry is one subject on which Joly is circumspect, because of her professional involvement. She does, however, express a hope that the SFO will not lose interest as it faces pressure to cut costs. Plans to save money by merging the SFO and other agencies into a new economic crime agency concern Joly. "I think it's a scandal. What sort of society are we creating? All over the world, the powerful don't want economic crime investigation."

Some of her most swingeing analysis is reserved for the Britain's failure to tame the forces in the Square Mile. "Your political and business elites are asking for sacrifice, asking to close down public services – you are cutting the number of public sector jobs by 400,000. Why don't your politicians have the courage to recover taxes in order to be able to pay for the schools, the teachers, the hospitals? To me, tax is civilization. It is the basis of your social contract."

The decision by the justice secretary, Kenneth Clarke, to delay implementing the Bribery Act, passed in the dying days of the Labour government, enrages Joly. "We have been waiting for that how long? 20 years?" Contrary to claims by CBI boss John Cridland that the act was "not fit for purpose", Joly insists it would merely bring Britain into line with standards elsewhere in Europe.

If Joly sometimes sounds like a revolutionary firebrand, she says it was not always so. "I used to be a very lawful citizen thinking that the laws were good and that if they were applied correctly that there was a kind of direction of the world, that people were taking care. Then15 years ago I started to understand that it's not true, that it's a kind of theatre, that we were all fooled, and that there are a lot of crooks out there."

Disapproval

Despite living and working in France for decades, her perspective remains self-consciously that of an outsider. Norwegian by birth, she came to France, aged 20, as an au pair called Gro Farseth. To initial family disapproval, she fell in love and married the eldest son of the family that employed her, Pascal Joly, and settled in her adopted country using her middle name, Eva, which is easier to pronounce in French.

She initially took secretarial work to help support her husband through medical school, but went on to take night classes in law. This laid the ground for a steady rise up the French legal ladder and she became a magistrate at the age of 38. She later became one of the first people not to have attended the prestigious École Nationale d'Administration to break into the profession's senior ranks.

"I thought that if I told people [what she knew] – writing books, giving interviews – the world would change. I thought it was only because people didn't know. And then I think the most terrible thing is that, even when things are said, they don't change. Then I thought that I must become a politician and really express these concerns. I've been doing a lot of things in my life and I can interconnect knowledge from different sectors."

When the taxi pulls up at the Gare du Nord, the interview ends. The African-French driver unloads Joly's suitcase from the boot, clasps her hand, says he is an admirer of her activism and wishes her well: "Bon courage. Bon courage."